Intrusion detection is becoming more important, both from a military standpoint and from a commercial activity. Military bases and installations, factories, power stations, stores, and even private homes are being protected by sophisticated electronic surveillance systems which are capable of monitoring the periphery of the facility so as to warm against unwanted intrusion.
One type of intrusion detection which has become more important in this security conscious world is that where a sensing element is concealed in the ground around the outer edges of an area to be protected. The sensing element of whatever type is adapted to monitor the movement of persons or objects approaching the outer edge, usually producing a signal indicating such an intrusion.
To enhance the value of any intrusion detection system, it is desirable that the sensor be able to detect more than one form of intrusion. Specifically, magnetic intrusion is normally detected, such as, for example, by detection of a mass of metal which breaks through or encroaches on a magnetic field. Objects which might be detected by these devices are obvious metal products such as vehicles, weapons, tools, and the like. Examples of patents which disclose sensors capable of detecting intrusion by metallic objects are U.S. Pat. No. 3,747,036 and U.S. Pat. No. 3,754,223.
While detection of intrusion by the aforementioned type is likely to give a reasonable degree of assurance, additional protection is desired if it can be achieved without greatly increasing cost and technical difficulties. Obviously, watchmen or sentries can be placed around an area to be guarded, but this is an expensive and not always reliable safeguard. Remote control cameras and other visual sensors bring about additional costs. Intrusion detection sensors have been developed which permit the use of already existing magnetic apparatuses. These intrusion detection systems detect intrusion by recognition of near-field surface disturbances so as to intercept and detect non-metallic objects moving across the line to be guarded. An example of a patent which discloses sensors capable of detecting near-field surface disturbances is U.S. Pat. No. 4,001,745.
Nevertheless, if the area to be protected is substantial in the size, or if the surface under which the sensor is to be buried varies in density or compactness, difficulties can arise in providing a sensor capable of completely surrounding the area to be detected. For example, when a relatively large area is to be surrounded by an intrusion detection sensor, it would be practical to attach a plurality of sensors together in series, both for reasons of ease of manufacture and for convenience in installation. However, at that point where two intrusion detection transducers are joined together, the sensitivity of the line sensor may be substantially less since, at the connecting area, there is no intrusion detection mechanism. Accordingly, in an intrusion detection system which uses a plurality of intrusion detection transducers, several weak points in the line may exist at these connecting points.
Oftentimes, when a relatively large area is being protected by an intrusion detection line, the area is comprised of more than one type of surface. Specifically, for example, part of the perimeter being protected could be comprised of ordinary dirt or earth while other areas might be paved or covered over by materials which yield little to surface loads when compared to the earth or other portions of the perimeter. It is impossible to provide an increased sensitivity in these areas of low seismic or pressure transmission without simultaneously increasing the sensitivity of the less sensitive area to a point where remote or background disturbances cause unwanted false alarms.
Accordingly, it is an object of this invention to provide a plurality of transducers which can be connected together without danger of significant insensitivity at their junctions.
Another object of this invention is to provide a means for increasing the relative sensitivity of one transducer in a line of plurality of transducers to permit adjustment for various media in which the line transducer is buried.
Other objects will appear hereinafter.